US2938233A - Pellet formation - Google Patents

Description

May 31, 1960 H. NACK ET AL PELLET FORMATION Filed Sept. 21, 1955 INVENTOR. Hermon Nock George F. Sochsel ATTORNEYS.

2,938,233 PELLET FORMATION Herman Nack, Columbus, and George F. Sachsel, Worthington, Ohio, assignors, by mesne assignments, to G & A Laboratories, Inc., Savannah, Ga., a corporation of Georgia Filed Sept. 21, 1955, Ser. No. 535,721

6 Claims. (Cl. 18-472) This invention relates to a method of forming pellets coated with a finely divided material wherein the coating material may be of the same composition as the pellets or of a composition difierent from the pellet material.

Various methods are known for the formation of pellets coated with a finely divided material. Of the methods generally known, there is the prilling process which, as commonly practiced, involves the freezing of drop lets of a melt or supersaturated solution in a stream of air that is cooler than the prilling tower feed. Other known methods include the dropping or sprinkling of the melt or solution onto a moving mass or bed of finely divided particles of the coating material, which particles are substantially stagnant with respect to each other, or into a revolving drum containing the coating material. The prilling system requires the control of many critical variables including air flow turbulence and the dropping distance. In addition, it is usually necessary to coat the pellets produced by the prilling process after they leave the prilling tower by means of an additional coating step. The disadvantages of the stagnant bed method of pelleting and coatingmaterials are the inefilcient cooling of the pellets and the deformation of the pellets upon impact with the mass of finely divided particles of material, which particles are substantially motionless with respect to each other. Pellets produced by sprinkling or dropping the material to be pelleted into a mass of finely divided coating material in a rotating drum, although substantially spherical in shape, are nonuniform in size, therefore requiring an additional step of classification when uniform sized pellets are desired. In contrast, by means of this invention the pelletizing and coating steps are carried out in one operation to produce substantially spherical pellets of essentially uniform size. Other objects and advantages of the invention, such as high heat-transfer rates between the bed or mass of finely divided particles of coating material and the material to be pelleted, will appear from the following description and appended claims when considered in connection with the accompanying drawings in which Fig. 1 is a diagrammatic view partly in section of a pelleting and coating apparatus showing also means for recirculating the coating material; and

Fig. 2 is a sectional view of the pelleting and coating mixer taken on the line 2-2 of Fig. 1.

In general, this invention comprises supplying a finely divided or powdered coating material container having situated therein rotary means shown in the drawings as spirally shaped mixing blades 12 rotated by the drive shaft 11. The amount of coating material supplied to the container is sufiicient so that by adjusting the speed of rotation of the rotary mixing means the surface level of the rapidly revolving mass of coating material rises to just below the arc of travel of the mixing means above the axis of rotation. Rotation of the mixing means imparts a revolving movement to the mass with beating or mixing of air into the mass while also causing the finely divided particles of material to randomly move with re- 2,938,233 Patented May 31, 19,60

spect to each other, thereby conditioning the mass into a state of fluidity. The mass of finely divided coating, material is a revolving internally agitated mass of finely divided particles having an appearance of a semi-fluid solid. With the coating material in this condition, droplets of the material to be pelleted are dropped or sprayed into the coating material in a direction transverse to the direction of the aerated or gas-expanded masss circum- :ferential revolution. While being mixed and revolved with the mass of revolving internally agitated coating particles the droplets are coated with the coating material and cooled. The pellets removed from the revolving mass of coating material are substantially spherical in shape and of uniform size, the size being determined by the initial size of the droplets of the material to be pelleted supplied to the mass of coating material.

The particular apparatus shown in the drawings is primarily useful for the pelleting of thermoplastic materials, such-as rosin, and the coating of the pellets with such ma terials as finely divided clay. The process is especially useful in the production of pellets of iodine disproportionated rosin having free flowability and good storageability imparted to the pellets by means of coatings of attapulgite clay according to the invention disclosed and claimed in US. Patent No. 2,797,211. Although the process of this invention is specifically described with respect to the use of the iodine disproportionated rosin and attapulgite clay as a coating material in the operation of the apparatus shown in the drawings, it is to be. understood that bther materials to be pelleted and other coating materials may be used.

Referring to Figs. 1 and 2, the numeral 10 refers to an j'open-top mixing container having situated therein a rotary mixer or blender comprised of a drive shaft 11 rotated by some suitable means not shown and having supported thereon by support means 13 spiral. ribbon mixing blades 12. Conveniently situated in the bottom of the container 10 is an opening 14 surrounded by the wall. 15. In the opening 14 is a gate valve 16. By opening gate valve 16 portions of the clay mass 33 and the solidified and coated pellets 32 are removed from the container and dropped onto the screen 19 upon which the solidified and coated pellets 32 are separated from the excess clay and removed to a suitable container 24 through the opening 23 in the enclosure wall 22. The excess clay separated from the pellets is passed to a clay storage tank 20 and is recycled to the container 10 through the pipe 25 and by means of the conveyors 26 and 28 connected by the pipe 27 and through the hopper 29. The enclosure walls 21 and 22 encase the opening 14, the screen 19 and the storage tank 20 so as to prevent the spread of dust resulting from the screening operation. The numeral 17 designates a pipe or conduit leading from a suitable source of molten rosin, the source of the rosin not being shown. The section of the pipe 17 situated above the open top of the container 10 contains a multiplicity of nozzles designated by the numeral 18. The rosin is emitted from the nozzles 18 in the form of molten drop lets or in the form of a stream which breaksup into droplets before contacting the clay mass in the container 10'. The size and form of the molten rosin pellets may be controlled by the shape of the nozzles 18 and the size of the nozzle openings.

The. amount of clay in the container 10 is controlled sothat by adjusting the speed of rotation of the mixing blades 12 the surface level 33 of the rapidly moving clay mass 30 rises to just below the arc of travel of the mixing blades 12 above the axis of rotation of the blades. The rise in the surface level of the clay mass is accompanied with the beating and blending of air into the clay mass by the mixing blades. The air blended into the mass of clay acts to expand the mass and, in a sense, acts as distance of only 7.5 to Sinches.

a lubricant for the clay particlesenhancing the random movement of the particles with respect to each other while the mass is revolved, thereby presenting a revolving internally agitated .mass of clay conditioned into a state of fluidity. The aerated or air-expanded revolving mass of clay quickly cools the molten rosin droplets without agglomeration. The impact resistance of the surface of the aerated or air-expanded mass of clay is decreased, practically eliminating deformation of the rosin droplets on contact with surface of the mass. As the rosin droplets revolve with the clay mass, they are cooled and coated with a thin adherent film of the clay, resulting in the desired clay coated rosin pellets. If the amount of clay in the container is such that the surface level 33 of the revolving clay is substantially above the arc of travel of the rotating mixing blades 12 the pellets produced are elongated, stringy, and friable. Thus, with the-surface level of the revolving mass of clay about 2 to 2.5 inches above the top of the arc of travel of the mixing blades 12, the surface of the clay mass moves slowly and with an undulating movement, the undulating movement corresponding to the passage of the mixing blades underneath the surface of the clay mass With the surface level of the revolving clay mass about 1 inch above the arc of travel of the mixing blades, the linear speed of the surface of the clay. mass increases but still with an undulating motion. In this latter case, the linear speed of the clay surface was measured and found to be approximately to 80 inches per minute. With the level of the revolving clay mass reduced to about 1 to 1.5 inches below the arc of travel of the mixing blades, the linear speed of the clay surface increases noticeably and approaches the mixing blade linear speed which was calculated to be about 1800 inches per minute. In this instance, the volume of the .clay mass upon being revolved increased by approximately 20 percent over the volume of the clay mass before revolving and air-expanding the mass by the mixing operation. Thus, the air-expanded clay mass represented arevolving fluidized or semifluidized solid having a volume ratio of about five parts solid to one part gas. The results of the runs under these. described conditions are tabulated below. In each of the runs the speed of rotation of the mixing blades was constant.

opp Run Clay Level Distance Remarks No. (in inches) (in 7 inches) 1 22.5 above are 4. 55 roduct entirely strings and flat of travel. 1 1 particles. 2. 1 above are of 6 Pellets irregularly'shaped, product travel. contains numerous strings. 3. 1-1.5 below are 7. 58 Pelletsround anduniforrn, product of travel. 7 contains only few stringy partlc es. 4..." 1 above are of 12 Pellets irregularly shaped with travel. strings. V 5. 1-1.5 below arc 13. 5-14 Pellets round and uniform, product of travel. contains only few strings.

The importance of the faster clay movements and aeration or gas expansion obtained by means of reducing the surface level. of the revolving clay mass below the arc of travel of the mixing blades above the axis of rotation is clearly shown in this table. Examination of this table also shows that the dropping distance of the droplets to the clay is not a critical factor in obtaining the clay coated rosin pellets. This is particularly demonstrated by the results-of Runs 3, 4, and 5. '-In Run 3 the clay level was below the, arc of travel of the mixing blades and a good uniform product was obtained with a dropping In Run 4 with the clay level above the arc of travel of the mixing blade poor results. were obtained with a dropping distance of 12 inches. However, the good results were again obtained in Run 5 with a dropping distance of 13.5 to 14 inches with the surface level of the revolving clay mass the same -it is desired to produce.

as in Run 3, thus showing that the dropping distance in Run 4 and also in Run 1 and Run 2 was not the cause of the poor results.

By this process it is possible to produce clay coated pellets of such materials as rosins, asphalt, resins, waxes and it is also useful in theproduction of coated pellets of hygroscopic materials such as ammonium nitrate. The finely divided coating material, where suitable, may be of the same composition as the material being pelleted. While reference has been made to finely divided coating material, it is, of course, understood that this has reference to either a powdered material or to particles of larger sizes but which are nevertheless smaller than the pellets In the pelleting and coating of some materials requiring large amounts of heat transfer from the pellets to the mass of coating material, some means of cooling the coating material may be desirable. Cooling of the coating material may be easily accom plished during recycling such as bythe passage of a cooling medium through -a jacket cooler suitably situated about the conveyors 26 and 28 in Fig. 1. Alternatively, the container 10 could be jacketed also for the passage of a cooling medium. From the foregoing it should be understood that the invention is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings since the invention is capable of being practiced or carried out in various ways.

What is claimed is:

1. In a method of forming spherical, coated pellets, in which method a bladed mixer means is rotated in a bed of particles of finely divided solid coating material having liquid droplets of a solidifiable material fixed therewith, and while a gas is mixed in said bed during rotation to expand the volume of said bed,to produce an internally, rotationally agitated mass revolving in the direction of said. rotating bladed mixer means, and with said particles moving randomly with respect to each other at a speed to condition said mass to a state of fluidity, and with said particles being at a temperature below the temperature of the liquid droplets, and in which the solidified droplets are removed therefrom as the coated pellets, the steps of: introducing the solidifiable material in liquid form through the top surface of said mass; and maintaining the top surface of said mass during rotation of the mixer means above the axis of rotation of the mixer means and below the top of the arc of rotation of said mixer means. 7

2. A method for preparing spherical pellets of a solidifiable material coated with finely divided particles of a solid coating material, said method comprising: rotating a bladed mixer means in a mass of the finely divided particles to mix a gas in said mass, and expand the volume of said mass during said rotating, to revolve said mass of finely divided particles and agitate said particles, and to condition said mass to a state of'fiuidity whereby said particles move randomly with respect to each other within the mass; maintaining the, top surface of said mass during said rotating above the axis of rotation of said mixer means and below the top of the arc of rotation of said mixer means; introducing liquid droplets of the solidifiable materialint'o the surface of the mass during said rotating with said'mass being at a temperature below the solidification point of the solidifiable material; and removing said pellets as solidified droplets of the. solidifiable material coated with tihefinely divided particles of the solid coating material.

3. The method of claim 2 in which'the solidifiable material is a thermoplastic material.

4. The method of claim,2 in which the. solid. coating material is clay and the. solidifiable material is a thermoplastic material. V I, .5 Y

5. The method of. claim 2 in which the solid coating material is clay and the solidifiable material is rosin.

.6. A method for preparing spherical pellets of rosin coated with finely divided particles of clay, said method comprising: supplying a mass of finely divided particles of clay to a mixer having mixing blades revolving to mix air at atmospheric pressure with said mass, to expand the volume of said mass, to revolve said mass of finely divided particles and agitate said particles, and to condition said mass to a state of fluidity whereby said particles move randomly with respect to each other within the mass; maintaining the top Surface of the revolving mass of clay above the axis of rotation of said revolving blades and said revolving blades having an arc of travel 10 during rotation that extends partially above the top surface; introducing liquid droplets of molten rosin into the air-expanded, revolving, conditioned clay mass in a direction transverse to the direction of the circumferential revolution of the mass, said mass being at a temperature 15 2,726,852

below the solidification point 'of the rosin; removing a portion of the solidified droplets and mass of the clay; separating solidified droplets as said droplets of rosin coated with finely divided particles of clay from clay in the removed portion; and returning the clay separated from said portion to the mixer.

References Cited in the file of this patent UNITED STATES PATENTS 857,756 Reddy June 25, 1907 2,457,962 Whaley Jan. 4, 1949 2,484,792 Mollring Oct. 11, 1949 2,574,357 Stammer et a1 Nov. 6, 1951 Somrner Dec. 13, 1955 UNITED STATES PATENT OFFICE CERTIFICATE 0F 'CQRRECTMN Patent N00 2,988,233 May 31 1960 Herman Nae k et all It is hereby certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 33 for "fixed" read mixed Signed and sealed this 15th day of November 19600 (SEAL) Attest:

KARL H AXLINE ROBERT C. WATSON Attesting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTEUN Patent N0o 2,938,233 May 31, 1960 Herman Nack et all,

It is hereby certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4L line 83, for "fixed" read mixed Signed and sealed this 15th day of November 1960,

( SEAL) Attest:

KARL Ha AXLI NE ROBERT C. WATSON Attesting Oflicer Commissioner of Patents

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